1. Field of the Invention
The present invention relates to LCD display technologies.
2. Related Art
Liquid crystal displays (LCD displays) are known in the field. An exemplary LCD display includes a number of pixel elements, usually arranged in a matrix formation, where each pixel element may be controlled individually to either emit or not emit light. By selectively controlling each pixel, a moving image may be created.
The LCD display may be formed by a liquid crystal provided in between two layers, where each layer may be formed by a plurality of sublayers. One of these sublayers is formed by liquid crystal elements that may be controlled by electrode layers (Indium Tin Oxide layer or ITO layer) arranged to address specific regions (pixels) of the liquid crystal element by applying a voltage to these specific regions. By applying a voltage (possibly zero volts) the orientation of the molecules of the liquid crystal may be controlled in such a way that the liquid crystal element may be in either an opaque state or a transparent state or in one or more semi-transparent states.
The liquid crystal and the electrodes are positioned in between two polarizing filters. Light travelling through the stack of liquid crystal display may be blocked or let through depending on the orientation of the liquid crystal molecules (depending on the voltage applied to the liquid crystal) of each pixel and the orientation of the polarizing filters.
In an example, the polarizing filters are positioned with their polarizing directions perpendicular with respect to each other. The liquid crystal is positioned in between two ITO layers. The ITO layers form electrodes to address pixels and apply a desired voltage to the liquid crystal.
The ITO layer comprises a liquid crystal alignment sublayer, for instance a sublayer of polyimide, facing the liquid crystal, treated to align the liquid crystal molecules in a certain direction. This treatment may be applied by rubbing (a cloth made of cotton) in a certain direction. The direction of alignment of the crystal molecules is defined by the direction of rubbing, which corresponds to the polarizing directions of the polarizing filters. The rubbing action may leave certain lines.
In case no voltage is applied to a given pixel, the molecules will align with the rubbing lines, and will therefore be arranged in a helical structure, or twist. Light passing through the first polarizing filter is rotated by this helical structure as it passes through the liquid crystal, allowing it to pass through the second polarizing filter.
However, once a voltage is applied to a certain pixel, with the electrical field lines running from one electrode sublayer to the other electrode sublayer, the helical structure will be disturbed under influence of the electrical field lines. As a result, no light can pass through this pixel as it is blocked by the second polarizing filter. This is also referred to as a ‘normally not black’ LCD display.
It will be understood that many variations to this example are possible and put to practise. For instance, by choosing a different relative orientation of the polarizing filters a ‘normally black’ LCD display DI may be created, meaning a LCD display in which pixels appear substantially black when no voltage is applied.
In order to create a color LCD display, each pixel may be divided into sub-pixels that may individually be addressed. Each pixel may be divided according to the RGB-arrangement, such that each pixel may be divided in a red, a green and a blue sub-pixel, as will be known to a skilled person. The colors are added to the arrangement by adding a separate color filter sublayer, for instance somewhere in between the polarizing sublayers. The color filter sublayer may be a matrix of adjacent color filters.
LCD displays are also used in so-called double layer super twisted nematic (DSTN) displays, in which a compensating sublayer is provided in addition to a LCD display, to provide a sharper image. The compensating sublayer is formed by an LCD display, without pixels formed therein, and is referred to as a passive LCD display (also known as compensator cell in DSTN displays). Therefore, the ITO layers of such a passive LCD display are unstructured, in contrast to a ‘normal’ LCD display, also referred to as the active LCD display. The passive LCD display may also be regarded as an LCD display with only one pixel.
For both the active and the passive LCD displays, the liquid crystal is captured in between two ITO layers. Spacers are provided in between the two ITO layers to keep the ITO layers at a predetermined distance with respect to each other. The spacers are formed by a plurality of grains or powder, such as a plurality of spheres or substantially round elements having a diameter of e.g. 4-6 μm.
However, it is observed that the spacers may move through the display, for example, under the influence of vibrations. Vibrations may occur during an ultrasonic cleaning process during manufacturing, where the frequency of the ultrasonic bath can cause moving spacers, or during the actual use of the LCD display. For instance in application in the automotive industry, vibrations may be caused by the engine of a car.
In the known designs, the barriers that limit the movements are the rubbing lines and gaps provided in the ITO layer. This may lead to clustering of spacers along the barriers. Clustered spacers may result in visible lines in the LCD display, disturbing the view of an observer. The yield figures from the production of LCD's show that visible clustering of spacers provide a significant contribution to the reject rate, especially in DSTN-displays for automotive applications. Therefore, it is desirable to provide an LCD display that is less sensitive for moving spacers.